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<h1><a name="SECTION005100000000000000000"></a><a name="truth"></a>
<br>
3.1 Truth Value Testing
</h1>

<p>
Any object can be tested for truth value, for use in an <tt class="keyword">if</tt> or
<tt class="keyword">while</tt> condition or as operand of the Boolean operations below.
The following values are considered false:
<a id='l2h-148' xml:id='l2h-148'></a><a id='l2h-149' xml:id='l2h-149'></a><a id='l2h-150' xml:id='l2h-150'></a><a id='l2h-151' xml:id='l2h-151'></a><a id='l2h-158' xml:id='l2h-158'></a>

<p>

<ul>
<li><code>None</code>
        <a id='l2h-153' xml:id='l2h-153'></a>
<p>
</li>
<li><code>False</code>
        <a id='l2h-155' xml:id='l2h-155'></a>
<p>
</li>
<li>zero of any numeric type, for example, <code>0</code>, <code>0L</code>,
        <code>0.0</code>, <code>0j</code>.

<p>
</li>
<li>any empty sequence, for example, <code>''</code>, <code>()</code>, <code>[]</code>.

<p>
</li>
<li>any empty mapping, for example, <code>{}</code>.

<p>
</li>
<li>instances of user-defined classes, if the class defines a
        <tt class="method">__nonzero__()</tt> or <tt class="method">__len__()</tt> method, when that
        method returns the integer zero or <tt class="class">bool</tt> value
        <code>False</code>.<a name="tex2html6"
  href="#foot4172"><sup>3.1</sup></a>
<p>
</li>
</ul>

<p>
All other values are considered true -- so objects of many types are
always true.
<a id='l2h-159' xml:id='l2h-159'></a>

<p>
Operations and built-in functions that have a Boolean result always
return <code>0</code> or <code>False</code> for false and <code>1</code> or <code>True</code>
for true, unless otherwise stated.  (Important exception: the Boolean
operations "<tt class="samp">or</tt>"<a id='l2h-156' xml:id='l2h-156'></a> and "<tt class="samp">and</tt>"<a id='l2h-157' xml:id='l2h-157'></a> always
return one of their operands.)
<a id='l2h-160' xml:id='l2h-160'></a>

<p>
<br><hr><h4>Footnotes</h4>
<dl>
<dt><a name="foot4172">...False.</a><A
 href="truth.html#tex2html6"><sup>3.1</sup></a></dt>
<dd>Additional 
information on these special methods may be found in the
<em class="citetitle"><a
 href="../ref/ref.html"
 title="Python Reference Manual"
 >Python Reference Manual</a></em>.

</dd>
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